The present invention relates to a phase change type optical recording material, and its fabrication method.
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for rewriting or overwriting. One typical rewritable optical recording medium is of the phase change type wherein the recording layer is irradiated with laser beam to change its crystallographic state whereupon a change of reflectance by the crystallographic change is detected for reading. Optical recording media of the phase change type are of great interest since they can be overwritten by modulating the intensity of a single light beam and the optical system of the drive unit used for their operation is simpler as compared with magneto-optical recording media.
Most optical recording media of the phase change type use chalcogenide materials such as Ge--Te, and Ge--Sb--Te base materials which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state.
In recent years, it has also been proposed to use compounds known as chalcopyrites.
Chalcopyrite compounds are extensively investigated as compound semiconductor materials, and applied to solar batteries and the like. The chalcopyrite compounds are composed of Ib--IIIb--VIb.sub.2 or IIb--IVb--Vb.sub.2 as expressed in terms of the Groups of the Periodic Table and have two stacked diamond structures. The structure of chalcopyrite compounds can be readily determined by X-ray structural analysis and their basic characteristics are described in Physics, Vol. 8, No. 8 (1987), page 441, Denki Kagaku (Electrochemistry), Vol. 56, No. 4 (1988), page 228, and other literature.
Among the chalcopyrite compounds, AgInTe.sub.2 is known to be applicable as a recording material by diluting it with antimony or Sb or bismuth or Bi. The resulting optical recording media are generally operated at a linear velocity of about 7 m/s. See JP-A's 3-240590, 3-99884, 3-82593, 3-73384 and 4-151286.
In addition to these phase change type optical recording media using chalcopyrite compounds, JP-A's 4-267192, 4-232779 and 6-166268 disclose phase change type optical recording media wherein an AgSbTe.sub.2 phase forms upon the crystallization of a recording layer.
When information is recorded on a phase change type optical recording medium, the recording layer is irradiated with laser beam of power (recording power) high enough to bring the recording layer to a temperature higher than the melting point thereof. The recording layer is melted at spots with the recording power applied thereon, and then quickly cooled so that recorded marks of amorphous nature can be formed. When the recorded marks are erased, on the other hand, the recording layer is irradiated with laser beam having such a relatively low power (erasing power) as to bring the temperature of the recording layer to a temperature higher than that the crystallization temperature thereof but lower than the melting point thereof. The recorded marks with the erasing power applied thereon go back to the amorphous state because they are slowly cooled down after heated to the temperature higher than their crystallization temperature. If recording power and erasing power are applied to the medium with a modulation of the intensity of a single light beam, overwriting is then possible.
However, a problem with a phase change type of optical recording media based on Ge--Sb--Te, AgInTe.sub.2, and AgSbTe.sub.2 systems is that C/N drops due to signal degradation such as erasure rate decreases or jitter increases due to repeated overwriting. This in turn leads to another problem that the number of repeatedly overwritable cycles is limited to about 1,000.